1. Field of the Invention
The invention relates to photographic reproduction and more particularly to a method of making a variety of multilevel equidensity contour mappings and pseudo-colored versions of a black and white photograph.
2. Description of the Prior Art
Equidensity contour mapping is important in the art of optical data processing such as tomography (i.e., the diagnostic views of X-ray photographs), pattern recognition, and image enhancement. Prior photographic techniques producing a single multilevel equidensity contour mapping of a photograph required many steps, which depend heavily upon film characteristics and therefore produced results which tend to be inaccurate and uncontrollable. More accurate techniques, such as digital optical processing, require very expensive equipment such as a microdensitometer in combination with a digital computer. While these results tend to be much more accurate and controllable they are too expensive to meet the needs of most users.
It is therefore an object of this invention to provide a method for making a variety of high quality equidensity contour mappings of a photograph by means of a simple photographic step.
From research results in optometry, it has been concluded that the human visual system can discriminate simultaneously only 15 to 20 gray levels from a complex black and white image. However, if the same image is presented in full color, the visually distinguishable levels can be increased enormously, up to hundreds or even thousands of different levels. Because of this increased resolution, techniques have been developed to encode color on black and white images such as radiographic, radioisotope scanning, and electron microscopic images. This encoding enhances the possibility of recognition or detection of the details of the images. The mapping of the black and white intensities into the three primary colors, i.e., blue, green, and red, is called pseudo-color encoding.
In the past, a pseudo-color encoding has been achieved mainly through two methods: a sophisticated digital method and a relatively simpler photographic method. The digital method involves the use of a flying-spot scanner, computations for intensity-to-color assignment, and output-color production. The sequential point-wise readings of the original black and white image, and the control and evaluation of the luminance and the chromaticity of the output image are assisted by a digital computer. The method is highly flexible but is also quite expensive. The photographic method, on the other hand, using only an incoherent light source, is much less expensive. Three photographic masks of different densities are made which are able to transform selectively the original gray levels into the three primary colors. The masks are used for the purpose of isolating the intensity levels in the black and white picture. The technique of producing the masks, especially the green mask, demands an accurate control of the gamma of the film and therefore requires several photographic steps.
It is therefore a further object of the invention to provide a method of encoding color onto a black and white image.